In recent years there has been growing interest in the use of artificial materials, particularly materials formed from polymers, for use in implantable devices that come into contact with bodily tissues or fluids particularly blood. Some examples of such devices are artificial heart valves, stents, and vascular prosthesis. Some medical devices such as implantable stents which are fabricated from a metal have been problematic in fracturing or failing after implantation. Moreover, certain other implantable devices made from polymers have exhibited problems such as increased wall thickness to prevent or inhibit fracture or failure. However, stents having reduced wall thickness are desirable particularly for treating arterial diseases.
Because many polymeric implants such as stents are fabricated through processes such as extrusion or injection molding, such methods typically begin the process by starting with an inherently weak material. In the example of a polymeric stent, the resulting stent may have imprecise geometric tolerances as well as reduced wall thicknesses which may make these stents susceptible to brittle fracture.
A stent which is susceptible to brittle fracture is generally undesirable because of its limited ability to collapse for intravascular delivery as well as its limited ability to expand for placement or positioning within a vessel. Moreover, such polymeric stents also exhibit a reduced level of strength. Brittle fracture is particularly problematic in stents as placement of a stent onto a delivery balloon or within a delivery sheath imparts a substantial amount of compressive force in the material comprising the stent. A stent made of a brittle material may crack or have a very limited ability to collapse or expand without failure. Thus, a certain degree of malleability is desirable for a stent to expand, deform, and maintain its position securely within the vessel.
Accordingly, it is desirable to produce a polymeric substrate having one or more layers which retains its mechanical strength and is sufficiently ductile so as to prevent or inhibit brittle fracture, particularly when utilized as a biocompatible and/or bioabsorbable polymeric stent for implantation within a patient body.